Various portable electronic devices may adopt various charging protocols. For instance, the main charging protocols adopted by the portable electronic devices currently in market may include Apple's voltage dividing charging mode, BC 1.2 dedicated charging port (“DCP”) mode, SAMSUNG 1.2V/1.2V charging mode and Quick Charging (“QC”) 3.0 mode. Besides these main charging protocols, there of course exist other charging protocols such as USB PD and MTK quick charging etc. A charger or a power adaptor should provide appropriate charging protocol that matches the portable electronic device it intends to charge before the charger/the power adaptor actually provides energy to charge the portable electronic device. Otherwise, the portable electronic device cannot be charged or can only be charged with very small charging current and thus takes a very long time to be charged up. With the requirements for usage convenience and higher charging speed, a charging circuit provides multiple charging protocols with auto matching of charging protocol to the portable electronic device needing to be charged is desired.